<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>JS变量</title>
    <script>
        //定义变量
        var a = 3;
        alert(a);
        a = "abc";
        alert(a);

        //定义 number 类型
        var num = 1;
        var num2 = 1.2;
        var num3 = NaN;

        //删除到页面上
        document.write(num + "-------" + typeof(num) + "<br>");
        document.write(num2 + "-------" + typeof(num2) + "<br>");
        document.write(num3 + "-------" + typeof(num3) + "<br>");

        //定义 String 类型
        var str = "abc";
        var str2 = 'edf';
        document.write(str + "-------" + typeof(str) + "<br>");
        document.write(str2 + "-------" + typeof(str2) + "<br>");

        //定义 boolean 类型
        var flag = true;
        document.write(flag + "-------" + typeof(flag) + "<br>");

        //定义 null,undefined 类型
        var obj = null;
        var obj2 = undefined;
        var obj3;
        document.write(obj + "-------" + typeof(obj) + "<br>");
        document.write(obj2 + "-------" + typeof(obj2) + "<br>");
        document.write(obj3 + "-------" + typeof(obj3) + "<br>");
    </script>
</head>
<body>

</body>
</html>